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constexpr specifier (ya que C + +11)

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C++
 
Lenguaje C++
Los temas generales
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General topics
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Control de flujo
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Flow control
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Sentencias condicionales ejecución
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Conditional execution statements
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Instrucciones de iteración
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Iteration statements
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Ir declaraciones
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Jump statements
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Funciones
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Functions
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declaración de la función
lambda declaración de la función
función de plantilla
especificador inline
especificaciones de excepción (obsoleto)
noexcept especificador (C++11)
Excepciones
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Exceptions
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Los espacios de nombres
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Namespaces
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Tipos
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Types
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decltype specifier (C++11)
Los especificadores
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Specifiers
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cv especificadores
almacenamiento duración especificadores
constexpr especificador (C++11)
auto especificador (C++11)
alignas especificador (C++11)
Inicialización
Original:
Initialization
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Los literales
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Literals
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Expresiones
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Expressions
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representaciones alternativas
Utilidades
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Utilities
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Tipos
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Types
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typedef declaration
alias de tipo de declaración (C++11)
atributos (C++11)
Lanza
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Casts
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conversiones implícitas
const_cast conversion
static_cast conversion
dynamic_cast conversion
reinterpret_cast conversion
C-cast estilo y funcional
De asignación de memoria
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Memory allocation
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Clases
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Classes
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Clase propiedades específicas de la función
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Class-specific function properties
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Funciones especiales miembro
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Special member functions
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Plantillas
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Templates
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clase de plantilla
función de plantilla
plantilla de especialización
paquetes de parámetros (C++11)
Varios
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Miscellaneous
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Montaje en línea
 
  • constexpr - especifica que el valor de una variable o función puede ser calculada en tiempo de compilación
    Original:
    constexpr - specifies that the value of a variable or function can be computed at compile time
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[editar] Explicación

constexpr especificador declara que no es posible evaluar el valor de la función o variable en tiempo de compilación, que luego se puede usar donde sólo el tiempo de compilación expresiones constantes se permiten. constexpr implica const .
Original:
constexpr specifier declares that it is possible to evaluate the value of the function or variable at compile time, they then can be used where only compile time expresiones constantes are allowed. constexpr implies const.
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'constexpr las variables' debe cumplir los siguientes requisitos:
Original:
constexpr variables must satisfy the following requirements:
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  • debe ser inmediatamente construido o asigna un valor .
    Original:
    it must be immediately constructed or assigned a value.
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  • el constructor de parámetros o el valor que se asignará debe contener sólo valores literales, variables y funciones constexpr .
    Original:
    the constructor parameters or the value to be assigned must contain only literal values, constexpr variables and functions.
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  • el constructor utilizado para construir el objeto (ya sea implícito o explícito) debe satisfacer los requisitos de constructor constexpr. En el caso de constructor explícito, debe haber especificado constexpr .
    Original:
    the constructor used to construct the object (either implicit or explicit) must satisfy the requirements of constexpr constructor. In the case of explicit constructor, it must have constexpr specified.
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'constexpr funciones' debe cumplir los siguientes requisitos:
Original:
constexpr functions must satisfy the following requirements:
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  • no debe ser virtual
    Original:
    it must not be virtual
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  • su tipo de retorno debe ser LiteralType
    Original:
    its return type must be LiteralType
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  • cada uno de sus parámetros deben ser de tipo literal
    Original:
    each of its parameters must be literal type
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  • el cuerpo de la función debe ser eliminado o cesación de pagos o sólo contienen lo siguiente:
    Original:
    the function body must be either deleted or defaulted or contain only the following:
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  • declaraciones nulas
    Original:
    null statements
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  • static_assert declaraciones
    Original:
    static_assert declarations
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  • typedef declaraciones y declaraciones alias que no definen las clases o enumeraciones
    Original:
    typedef declarations and alias declarations that do not define classes or enumerations
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  • mediante declaraciones
    Original:
    using declarations
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  • mediante directivas
    Original:
    using directives
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  • exactamente una declaración return que contiene sólo valores literales, variables y funciones constexpr .
    Original:
    exactly one return statement that contains only literal values, constexpr variables and functions.
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'constexpr constructor' deberán cumplir los requisitos siguientes:
Original:
constexpr constructor must satisfy the following requirements:
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  • cada uno de sus parámetros deben ser de tipo literal
    Original:
    each of its parameters must be literal type
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  • la clase no debe tener clases base virtuales
    Original:
    the class must have no virtual base classes
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  • el cuerpo del constructor debe ser eliminado o cesación de pagos o sólo contienen lo siguiente:
    Original:
    the constructor body must be either deleted or defaulted or contain only the following:
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  • declaraciones nulas
    Original:
    null statements
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  • static_assert declaraciones
    Original:
    static_assert declarations
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  • typedef declaraciones y declaraciones alias que no definen las clases o enumeraciones
    Original:
    typedef declarations and alias declarations that do not define classes or enumerations
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  • mediante declaraciones
    Original:
    using declarations
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  • mediante directivas
    Original:
    using directives
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  • el constructor no debe tener un bloque de función-try
    Original:
    the constructor must not have a function-try block
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  • cada clase base y cada miembro no estático debe ser inicializado, ya sea en el initializer_list o inicializador llave-o-igual. Termine cada constructor en cuestión debe ser un constructor constexpr y todas las cláusulas de cada inicializador llave-o-igual debe ser una expresión constante
    Original:
    every base class and every non-static member must be initialized, either in the initializer_list or by brace-or-equal initializer. End every constructor involved must be a constexpr constructor and every clause of every brace-or-equal initializer must be a constant expression
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  • cada conversión implícita en cuestión debe ser una expresión constante
    Original:
    every implicit conversion involved must be a constant expression
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[editar] Palabras clave

constexpr

[editar] Ejemplo

Definición de una función que calcula factoriales constexpr y un tipo literal que se extiende literales de cadena

Original:

Definition of a constexpr function which computes factorials and a literal type that extends string literals

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#include <iostream>
#include <stdexcept>
 
// constexpr functions use recursion rather than iteration
constexpr int factorial(int n)
{
    return n <= 1 ? 1 : (n * factorial(n-1));
}
 
// literal class
class conststr {
    const char * p;
    std::size_t sz;
 public:
    template<std::size_t N>
    constexpr conststr(const char(&a)[N]) : p(a), sz(N-1) {}
    // constexpr functions signal errors by throwing exceptions from operator ?:
    constexpr char operator[](std::size_t n) {
        return n < sz ? p[n] : throw std::out_of_range("");
    }
    constexpr std::size_t size() { return sz; }
};
constexpr std::size_t countlower(conststr s, std::size_t n = 0,
                                             std::size_t c = 0) {
    return n == s.size() ? c :
           s[n] >= 'a' && s[n] <= 'z' ? countlower(s, n+1, c+1) :
           countlower(s, n+1, c);
}
 
// output function that requires a compile-time constant, for testing
template<int n> struct constN {
    constN() { std::cout << n << '\n'; }
};
 
int main()
{
    std::cout << "4! = " ;
    constN<factorial(4)> out1; // computed at compile time
 
    volatile int k = 8;
    std::cout << k << "! = " << factorial(k) << '\n'; // computed at run time
 
    std::cout << "Number of lowercase letters in \"Hello, world!\" is ";
    constN<countlower("Hello, world!")> out2; // implicitly converted to conststr
}

Output: 

4! = 24
8! = 40320
Number of lowercase letters in "Hello, world!" is 9
Obtenido de «http://es.cppreference.com/mwiki/index.php?title=cpp/language/constexpr&oldid=23394»